Attempts have been made to use heat-screening glass coated with a metal nitride (such as TiN, ZrN, and CrN) in a single glazing in view of the fact that it is superior in chemical resistance and wear resistance to heat-screening glass coated with a noble metal (such as Ag). For the heat-screening glass to have practical chemical resistance and wear resistance which permit it to be used in a single glazing, it should be provided with a protective film which is composed of an adequate material and has an adequate thickness and layer constitution. As the protective film used in the conventional heat-screening glass composed of a glass substrate and a heat-screening film formed thereon, those composed of SiO.sub.2 (as shown in JP-A-63-206333), tantalum oxide (as shown in JP-A-1-145535 and JP-A-2-164744), and oxide of zirconium boride (as shown in JP-A-2901 and JP-A-1-314163)have been disclosed. The term "JP-A" used herein means an unexamined published Japanese patent application.
The heat-screening glass provided with a protective film composed of SiO.sub.2 has good wear resistance and acid resistance owing to SiO.sub.2. However, the protective film has to be thicker to compensate for its poor alkali resistance. For example, an SiO.sub.2 protective film thicker than 1 .mu.m does not show noticeable change in external appearance even after corrosion by alkaline substances. The thicker SiO.sub.2 protective film takes a long time for its formation, which leads to the poor productivity of heat-screening glass.
The heat-screening glass provided with a protective film composed of tantalum oxide has good acid and alkali resistance. However, it lacks practically high wear resistance and becomes noticeably hazy when the protective film wears.
The heat-screening glass provided with a protective film composed of oxide of zirconium boride has excellent wear resistance and alkali resistance. However, it is poor in acid resistance. In addition, this protective film is difficult to be produced at a high speed in a stable manner by the direct current sputtering method which is a favorable method for coating a large glass plate. This disadvantage of the direct current sputtering is due to the fact that it employs a target of sintered zirconium boride which is not sufficiently conductive and contains pores which cause arcing during sputtering. Arcing gives rise to fine particles which stick to the film to form pinholes.